Abstract:The Miocene volcanic lava in Canmeishan area, belonging to Qiangbaqian rock province of Hoh Xil rock belt, occurs in north Tibet and is composed from early to late of basalt latite, latite and trachytic dacite, with the K-Ar whole-rock ages in the range of 12.81～14.51 Ma. The SiO2 contents of basalt latite, latite and trachytic dacite are 53.85%～56.02%, 59.22%～61.84% and 64.81%～67.70%, respectively. The rocks belong to the shoshonite series and are sub-aluminous: K2O+Na2O 6.21%～8.20%, K2O 3.17%～4.94%, K2O/Na2O=1.01～1.52, TiO2 0.77%～1.88%, ACNK=0.54～0.90 and KNA= 0.57～0.72. ΣREE 297×10-6 ～939×10-6and (La/Yb)N 45.1～127.4 point to intensely-concentratedLREE, with δEu being 0.73～0.95. TheLILE (K, Rb, Cs, Sr, Ba, U, Th) are highly concentrated, while the HFSE (Nb, Ta, Zr, Hf) are comparatively less concentrated. Geochemical characteristics and regional contrast indicate that the source ofMiocene volcanic rocks in Canmeishan area was EMⅡ-mantle. Harker diagrams of selected major oxides, Mg# versus trace element variation diagrams, La/Sm-La diagram and variation of Mg# values and ΣREE all show that basalt latites and latites belong to the comagmatic evolutionary series, and have obviously experienced a high degree of crystallization differentiation. The trachytic dacites belong to another magmatic series produced by partial melting of the rocks formed from the earlier evolved magma. It is thus concluded that the spatial-temporal scale of deep magmatism should be larger than that of the upper volcanic rocks in north Tibet in Cenozoic. Under the compressive tectonic regime, some mantle-derived magma was confined in the depth and turned into rock through cooling. Consequently, the space-time distribution of Cenozoic volcanic rocks in north Tibet must have been related to the evolution of the regional tectonic regime.
